The present invention relates to a thin film forming method and apparatus, and in particular, to a thin film forming method and apparatus capable of forming a quality thin film at a high velocity on a surface of a substrate having particularly a stepped portion.
Particularly in processes subsequent to aluminum wiring formation among processes of manufacturing a semiconductor integrated circuit, there is widely used a plasma CVD method capable of forming a film at a low temperature as a method for forming a silicon oxide film or a silicon nitride film.
FIG. 8 shows a construction of a thin film forming apparatus used generally for manufacturing a semiconductor integrated circuit. In FIG. 8, an upper electrode 52 and a lower electrode 53 are arranged in parallel with each other inside a processing chamber 41. A semiconductor substrate 10 is disposed on the lower electrode 53. The upper electrode 52 is provided with gas inlets 54 on a side thereof facing the lower electrode 53. The upper electrode 52 is connected to a high-frequency power source 55 for upper electrode use, while the lower electrode 53 is connected to a high-frequency power source 56 for lower electrode use.
By introducing TEOS (tetraethyl orthosilicate) and oxygen as a reaction gas from the gas inlets 54 into the processing chamber 41 while discharging a gas in the chamber 41 therefrom, and applying high-frequency powers to the upper electrode 52 and the lower electrode 53 with the processing chamber 41 maintained internally at an appropriate pressure, plasma is generated inside the processing chamber 41, and a silicon oxide film is formed on the substrate 10. Otherwise, when silane, nitrogen, and ammonia are used instead of TEOS and oxygen, a silicon nitride film can be formed.
Accordingly as semiconductor devices have been improved in their level of integration, there has been a growing demand for forming a film while assuring a good covering performance on a stepped portion having a great aspect ratio (a ratio of a height to a bottom length of the stepped portion). However, according to the aforementioned film formation by means of the plasma CVD apparatus, it has been difficult to form a film while assuring a good covering performance on a stepped portion as described above. When an insulating film 70 is deposited on such a stepped portion, a cavity referred to as a void 71 is disadvantageously formed as shown in FIG. 10.
In view of the above, there has been performed a complex process for forming an insulating film free of voids through a by the plasma CVD method, and several representative techniques will be described below.
FIGS. 11A through 11D show a sandwich method. In FIGS. 11A through 11D, on a stepped portion 72 as shown in FIG. 11A, firstly an insulating film 73 is deposited by the plasma CVD method as shown in FIG. 11B. Then, SOG (Spin On Glass: Glass coated by spinning) is coated, and thereafter baking is performed to form a SOG layer 74 as shown in FIG. 11C. For the coating of SOG, a special coater is used. Then, an insulating film 75 is deposited by the plasma CVD method as shown in FIG. 11D. The SOG layer has an excellent stepped portion covering performance, has an inferior film property as an insulating film. The above-mentioned fact is the reason for providing the sandwich structure in which the SOG layer is sandwiched between the plasma CVD films each having a good film property. In place of SOG, a resin such as a resist is sometimes employed. When a resin is employed, a sandwich structure is still required for the same reasons as in the case of SOG.
FIGS. 12A through 12F show an etch back method. In FIGS. 12A through 12F, on a stepped portion 76 as shown in FIG. 12A, firstly an insulating film 77 is deposited by the plasma CVD method as shown in FIG. 12B. Then, a sputter etching process is performed by means of oxygen or argon plasma as shown in FIG. 12C so as to remove shoulder portions 77a of the insulating film 77. It is to be noted that the sputter etching process is performed by a dry etching apparatus having two electrodes in a vacuum vessel. Then, an insulating film 78 is deposited by the plasma CVD method as shown in FIG. 12D. Formation of an insulating film formation free of voids has been completed through the above-mentioned process stages. However, when an improved flatness is required, a resin layer 79 such as a resist is formed on the insulating film 78 as shown in FIG. 12E. Finally, by effecting a dry etching process with an etch selectivity of the resin layer 79 to the insulating film 78 being is 1:1 as shown in FIG. 12F, a flat insulating film 78 is formed without leaving the resin layer 79.
Further, there is proposed an ECR bias CVD method capable of forming a flat thin film in a single process. In FIG. 13 showing an approximate structure of an ECR bias CVD apparatus, an appropriate gas is introduced into a vacuum vessel 81 while a gas in the vessel 81 is discharged, and while a microwave generated by a microwave generator 86 is radiated into a discharge tube 85 via a waveguide tube 87 with the discharge tube 85 maintained internally at an appropriate pressure. Meanwhile, a static magnetic field is generated inside the discharge tube 85 by causing a direct current to flow through a static magnetic field generating coil 88 provided outside the discharge tube 85. Then, due to an interaction between a microwave electric field and the static magnetic field, an electron cyclotron motion is excited for electrons inside the discharge tube 85, thereby generating plasma. It is to be noted that an electrode 82 is connected to a high-frequency power source 84 for electrode use so that the electrode can control energy of ions to be incident on a substrate 83.
FIGS. 14A through 14D show a thin film forming process which is carried out by means of the ECR bias CVD apparatus. In FIGS. 14A through 14D, when an insulating film is formed on a stepped portion 90 as shown in FIG. 14A, sputter etching is effected simultaneously with thin film deposition. Therefore, thin film formation progresses in a manner as shown in FIGS. 14B through 14D, thereby obtaining a flat insulating film 91 free of voids.
However, each of the thin film forming methods and apparatuses as described above have issues as follows.
In terms of structure, the sandwich method has the portion having an inferior film property such as SOG and resin in the insulating film. Therefore, a defect such as reduction of dielectric strength or deterioration of aluminum wiring may occur due to moisture contained in, for example, SOG, and an achievable flatness is inferior to the other methods. Therefore, it is inappropriate to apply this method to a stepped portion of a device where the aspect ratio is not smaller than one and the length of a bottom portion of the stepped portion is not greater than 0.8 .mu.m.
The etch back method, which does not, in the end, leave a portion having an inferior film property such as SOG and resin in the insulating film, has no disadvantage in terms of film quality. However, this method has a drawback that it has a small throughput because considerable time is required for the shaping process (shoulder removal process) performed by the dry etching apparatus having two electrodes in the vacuum vessel.
The ECR bias CVD method is more advantageous than the foregoing two methods in that film formation can be achieved in one process. However, since the processing with ECR plasma is performed in an intense magnetic field, this method has a fatal drawback that a thin insulating film (e.g., gate oxide film) fabricated in a device suffers a dielectric breakdown due to electrostatic charges. Therefore, though the method has been regarded as a promising one, practically the method has not been used in mass production. If the issue of dielectric breakdown is solved, the ECR plasma CVD apparatus is disadvantageously very expensive because the microwave generator 86 is expensive. Therefore, in order to reduce a substrate processing cost to a reasonable value, a substantial increase of film formation velocity is required.